The invention relates generally to the manufacture of printed wiring boards for electrical components and more particularly to photoprocessing techniques for printed wiring boards.
The ongoing integration and miniaturization of components for electronic circuitry has become a growing challenge to the limits of printed wiring board technology over the last twenty years. Printed circuit boards or printed wiring boards (PWB) as they are more accurately termed, play several key roles. First, the electrical components, such as specially packaged integrated circuits, resistors, etc., are mounted or carried on the surface of the flat usually sturdy card-like board. Thus, the PWB serves as a support for the components. Secondly, using chemically etched or plated conductor patterns on the surface of the board, the PWB forms the desired electrical interconnections between the components. In addition, the PWB can include a metal area serving as a heat sink.
Conductor patterns are typically formed by photoetching a copper foil clad epoxy fiberglass substrate. A photoresist layer is applied to the copper foil and patterned by exposure to untraviolet (UV) light projected through a mask, often referred to as "artwork". Areas exposed on the photoresist are polymerized. The unpolymerized areas are removed by a chemical solution leaving areas of copper, the desired conductor pattern, underneath the protective barrier of the remaining polymerized hardened photoresist. The exposed copper is then etched away and the remaining photoresist is chemically removed to expose the resulting conductor pattern. Alternatively, the photoresist can be patterned to form channels for electroless plating of conductor patterns. There are, of course, many variations on this procedure, but all of them require photo-patterning of the resist layer.
As the use of integrated circuits has grown, the higher density of interconnection terminals for inputs and outputs (I/O) has necessitated double-sided PWB's in which additional interconnections are made employing conductor patterns on the other side of the board.
Along with increased circuit integration, surface mount technology (SMT) has accelerated the densification of electronic circuitry. Surface mount devices (SMD) are applied directly to the surface of the PWB and soldered using vapor phase, infra-red (IR) or other techniques. SMT is revolutionizing the electronics manufacturing industry by reducing assembly cost by about 50%, increasing component density by over 40% and enhancing reliability. The array of terminals on SMD's has a higher density or finer pitch than those on conventional components. As each terminal still has to be properly electrically connected to the respective conductor on the board, registration of SMD's requires high resolution for the PWB conductor lines. Indeed, SMD circuitry has become so dense that double-sided boards cannot accommodate all of the needed electrical connections. Thus, multilayer PWB's have become the focus of attention and several competing technologies are evolving. Those technologies which rely on stacks or layers of conductor patterns have interlayer registration requirements in addition to the exacting line width and spacing of a conductor pattern in a given layer. Manufacturing very fine lines on the order of 3 to 5 mils in registration over four or more layers deep is very difficult.
To take fullest possible advantage of the benefits offered by the emerging SMT, new fabrication processes must be developed in the manufacture of substrates and boards. In the past, one of the problem areas in fabrication of PWB's has been the generation and use of artwork masters for patterning the photoresist layers. Using photographic film or glass plates poses inherent difficulties in stability, registration, transport and storage.
The elimination of artwork masters from the board fabrication process has long been an industry goal, one which fostered the development of high-speed UV laser plotters. Several machines are currently available; all are very expensive and in an early stage of development. These machines pattern the UV sensitive resist directly without artwork. Conductor patterns are designed using computer-assisted design (CAD) which digitizes the coordinates and dimensions of all of the paths and converts them to control signals for a UV laser x-y plotter. In addition to their very high cost, these systems have a number of limitations which become significant in fine line, high density work. Principal among these is the fact that UV sensitive resists are low contrast materials, requiring high levels of exposure energy. As a result, line edge resolution is limited. In order to achieve high plot speeds, these systems all operate in a raster scan mode. Raster scanning produces considerable edge irregularities which are very apparent in plotting angled lines. Limitations in accuracy and minimum line width are characteristic of these raster plotting systems. Another problem area in current systems is the short life expectancy of the laser source. A further problem with direct from CAD UV plotting of the photoprocessable layer is that it does not permit inspection before polymerization. If an error is made in the plot, the mistake is indelibly embedded in the UV sensitive layer. In the case of a resist, the board may be salvaged only by removing the entire resist layer and starting over after cleaning and baking the board free of moisture a second time. In the case of a UV plotted solder mask, a glitch in the pattern results in the entire panel being discarded.